1. Field of the Invention
The present invention relates to an electrolyte for an electrical double layer capacitor, and in particular, to an electrolyte for an electrical double layer capacitor possessing excellent withstand voltage and excelling in long term reliability and to an electrical double layer capacitor using the electrolyte.
2. Description of Background Art
An electrical double layer capacitor is a charge accumulation device comprising an electrical double layer formed from the interface of polarizable electrodes and an electrolyte.
When the electrolyte used in an electrical double layer capacitor possesses a high coefficient of viscosity or a low degree of conductivity, the internal resistance of the electrical double layer capacitor as a whole increases thereby decreasing the voltage of the capacitor during charging and discharging. Therefore, an electrolyte used in an electrical double layer capacitor is required to possess a low coefficient of viscosity and a high degree of conductivity. Also, since an electrical double layer capacitor is used in a sealed state for an extended period of time and is charged and discharged repeatedly, the electrolyte used in the capacitor is required to possess long term durability. Furthermore, in order to ensure durability of the electrolyte, the operating voltage of conventional electrical double layer capacitors has been limited to 2–2.7 V. An electrolyte for use in an electrical double layer capacitor that can be operated at a higher voltage has been desired.
Conventionally, in view of ensuring long term durability, electrolytes for use in electrical double layer capacitors comprise a tetraalkyl ammonium salt such as triethylmethylammonium tetrafluoroborate (hereinafter referred to as “TEMA-BF4”) dissolved in a cyclic carbonate such as propylene carbonate (hereinafter referred to as “PC”) (Japanese Patent Application Laid-open No. 2000-114105).
However, due to a high coefficient of viscosity of about 2 mPa·s at 30° C. possessed by cyclic carbonates such as PC, there has been a problem of the coefficient of viscosity increasing even more when tetraalkyl ammonium salts are dissolved in these cyclic carbonates. Furthermore, the low conductivity of these electrolytes increases the internal resistance of the electrical double layer capacitor.
On the other hand, since electrolytes obtained from linear carbonates have a low coefficient of viscosity as compared with those obtained from cyclic carbonates such as PC, the use of electrolytes possessing long term durability obtained from these linear carbonates has been anticipated.
However, tetraalkyl ammonium salts such as TEMA-BF4 generally have a low solubility in linear carbonates. Therefore, a capacitor possessing the benefits obtained from the use of a linear carbonate has yet to be realized.
In view of the above, the object of the present invention is to provide an electrolyte for use in an electrical double layer capacitor operating at a high voltage that has a low coefficient of viscosity, is highly conductive, and is excellent in long term reliability, and to provide an electrical double layer capacitor using the electrolyte.